An infection model of Giardia lamblia into adult mice has been developed. Only one isolate of nine studied consistently infected adult mice. Seven mice strains were readily infected but showed different courses of infections. The course of infection, changes in expressed VSP over time, and humoral responses were noted in a number of strains of mice as well as SCID and nude mice. Large numbers of trophozoites were present in the small intestines 1-2 weeks after inoculation. This was followed by a dramatic reduction in the numbers of trophozoites in the intestines. Two of three strains remained chronically infected, while a third was cured. SCID mice failed to show a reduction in the number of intestinal trophozoites over time. This model system allows the analysis of host- parasite interactions, especially those responses important in antigenic variation, maintenance of chronic infections, and cure. The metabolism of cysteine in Giardia was studied. Giardia could neither manufacture cysteine de novo or convert other sulfur-containing compounds to cysteine. Two mechanisms appeared responsible for uptake, one of which was non-saturable. Oxidation of cysteine to cystine occurs readily in culture and can be prevented by the use of a Cu++ chelating compound, batocuproine. ICF II, a component of Cohn fraction IV-1, is an essential growth- promoting factor of Giardia. Recombinant protein supported growth and cysteine uptake in the absence of serum, a necessary component for culture.